1. Field of the Invention
This invention relates to a method of preparing an improved photothermographic material by mixing certain components including a silver salt of certain heterocyclic thione compounds in a particular series of steps. One aspect of the invention relates to a photothermographic element or composition comprising certain 2-mercapto compounds with certain silver salts of heterocyclic thione compounds and other components to provide a photothermographic material with a spectral sensitizing dye, particularly a symmetrical or unsymmetrical thiosulfatoalkyl substituted benzimidazolocarbocyanine dye. Another aspect of the invention relates to a method of providing a developed image in the described material by merely heating the material to moderately elevated temperatures.
2. Description of the State of the Art
Photothermographic materials comprising photosensitive silver halide in association with a silver salt of certain heterocyclic thione compounds and an organic reducing agent with a spectral sensitizing dye are known in the art. These photothermographic materials and methods of their preparation are described, for example, in U.S. Pat. No. 3,785,830 of Sullivan, Cole and Humphlett, issued Jan. 15, 1974 and U.S. Pat. No. 3,893,860 of Sutton and Stapelfeldt, issued July 8, 1975. One problem which has been encountered with these photothermographic materials is the need to provide increased spectral sensitivity. A significant limitation on the photothermographic materials described is the lack of range of spectral sensitizing dyes which can be effectively incorporated in the photothermographic materials. It is believed that one reason for difficulty in spectral sensitizing photothermographic materials containing such a silver salt of a heterocyclic thione has been the lack of an effective method of preparing the material that provides improved retention of the spectral sensitizing dye on the photosensitive silver halide used in the composition. Several methods have been attempted in order to improve this disadvantage.
Another problem which has been encountered in the described photothermographic materials centers on spectral sensitizing a preferred photosensitive silver halide in the photothermographic materials. This preferred silver halide is silver iodide and silver bromoiodide comprising up to 30 mole % iodide.
The use of spectral sensitizing dyes in photosensitive silver halide materials, especially silver halide photothermographic materials, are well known in the art. A variety of mercaptan compounds, thiones and thioethers have also been used in a variety of photothermographic materials in combination with spectral sensitizing dyes. For example, U.S. Pat. No. 3,617,289 of Ohkubo et al, issued Nov. 2, 1971 indicates that a thermally developed light-sensitive material, one that is not based on photosensitive silver halide with a silver salt of certain heterocyclic thione compounds, can be stabilized by treating the material with a solution containing certain thiol or thione compounds. These materials can contain spectral sensitizing dyes. U.S. Pat. No. 3,831,186 of Masuda et al, issued Aug. 27, 1974 relates to a heat developable photographic material containing silver benzotriazole with photosensitive silver halide and a reducing agent with a toning agent which is a specific thiol compound. A sensitizing dye can be present in this heat developable material, such as one containing a thiohydantoin moiety. This photothermographic material, however, is not based on the combination of photosensitive silver halide with a silver salt of a certain heterocyclic thione compound. None of the references describe an answer to the problem of providing a wider variety of dyes which are useful for spectral sensitizing a photothermographic composition containing silver salts of certain heterocyclic thione compounds with photosensitive silver halide, especially silver iodide.
Heterocyclic thione compounds have been used for a variety of purposes in photographic materials, such as photothermographic materials. For example, in U.S. Pat. No. 3,669,672 of Shiba et al, issued June 13, 1972, a silver halide photographic emulsion is spectrally sensitized with a combination of at least one pentamethine type cyanine dye with a certain heterocyclic compound which is a thione. The combination of the pentamethine dye with the heterocyclic compound is indicated as providing supersensitization of the silver halide emulsion. U.S. Pat. No. 3,687,678 of Reister, issued Aug. 29, 1972 indicates the sensitization of photographic silver halide can be provided by a combination of at least one xanthylium, thiaxanthylium or selenaxanthylium dye with certain concentrations of a certain isocyclic or heterocyclic aromatic mercapto compound. Also, U.S. Pat. No. 3,839,041 of Hiller, issued Oct. 1, 1974 describes a variety of heterocyclic thione compounds which are useful as stabilizer precursors in photothermographic materials. These can be used in combination with spectral sensitizing dyes. Mercapto compounds and selenol compounds are useful for supersensitizing conventional photographic silver halide materials as described, for example, in British Specification 1,229,951 published Apr. 28, 1971. 2-Mercaptobenzothiazole and 2-mercaptobenzoxazole compounds have also been used in photographic materials containing a photographic silver halide emulsion with a silver halide developing agent. This is described, for example, in British Specification 1,049,054 published Nov. 23, 1966. This combination, however, is used for developing an image by contacting the photographic material with an alkaline activator in processing solutions or baths, not by processing with heat. U.S. Pat. No. 3,776,738 of Ohlschlager et al, issued Dec. 4, 1973 also indicates that silver halide photographic emulsions can be sensitized with a combination of cyanine dyes and a mercapto or selenol compound. 2-Mercapto substituted heterocyclic compounds have also been used in gelatino silver halide photosensitive emulsions in an attempt to provide optimum sensitization during digestion in emulsion preparation. U.S. Pat. No. 3,785,822 of Overman, issued Jan. 15, 1974 which relates to this use, however, does not indicate that this combination is useful with photothermographic materials based on silver salts of certain heterocyclic thione compounds. None of these references have provided a satisfactory solution to the problem of providing increased spectral sensitization for a photothermographic material containing photosensitive silver halide with a silver salt of certain heterocyclic thione compounds and a reducing agent, as described, for instance, in U.S. Pat. No. 3,893,860 of Sutton and Stapelfeldt, issued July 8, 1975.
There has accordingly been a continuing need to provide improved spectral sensitization of photothermographic materials comprising photosensitive silver halide in association with silver salts of certain heterocyclic thione compounds, as described. There has also been a continuing need to provide improved methods for preparing the described photothermographic materials containing spectral sensitizing dyes to provide the desired increased sensitivity.